It is known that p-xylene can be reacted with chlorine in the presence of a catalyst to produce a mixture of various ring-chlorinated compounds and isomers including 2,5-dichloro-p-xylene, a compound useful as an intermediate in the preparation of pesticides and polymeric materials. Such reactions may be effected in solution, in suspension or in the absence of a solvent. Thus, for example, in U.S. Pat. No. 2,412,389, it is disclosed that p-xylene may be chlorinated in the presence of a catalyst such as iron filings or ferric chloride, to produce mixtures of nuclear chlorinated materials which may then be separated by a series of steps to isolate some components of the mixture. However, the various compounds and isomers which may be produced in this manner are not of equal commercial importance or value. Although such a procedure may be of value in the production of mixtures of variously ring-chlorinated materials it provides little advantage as a direct chlorination process for the production of specific ring-chlorinated products.
A more specific process for the ring-chlorination of p-xylene, utilizing acetic acid as a solvent and carried out in the presence of catalytic amounts of ferric chloride or iodine is disclosed in U.S. Pat. No. 3,002,027. The process limits the ring-chlorination to the formation of mono- and dichlorination derivatives. The 2,5-dichloro-p-xylene isomer may be separated from the crude reaction product by pouring into water followed by re-crystallization of the precipitate from a suitable solvent such as an organic alcohol or acetic acid.
It is further known from U.S. Pat. No. 3,035,103, that p-xylene may be chlorinated, in a solvent, such as carbon tetrachloride, in the presence of a catalyst such as ferric chloride to yield a mixture of mono-, di-, tri-, and tetra- ring-chlorinated p-xylenes which may then be distilled and the dichloro-para-xylene fraction treated with a lower alkanol to recover the commercially desirable 2,5-dichloro-p-xylene.
Although the processes of the prior art are useful in the preparation of chlorinated xylenes and, with subsequent separation, the isolation of 2,5-dichloro-p-xylene, it will be appreciated that further improvements in the efficiency, economy of preparation and yield of the desired product, 2,5-dichloro-p-xylene, are nevertheless desirable.
In addition, it is known from U.S. Pat. No. 3,226,447 to Bing et al, that the ring chlorination of toluene, benzene, or chlorinated benzene may be advantageously carried out in the presence of a catalyst comprising a halide of iron, aluminum or antimony and a co-catalyst consisting of an organic sulfur compound characterized by divalent sulfur, to provide a chlorinated product wherein the yield of para-chloro isomer is substantially increased. It is further disclosed by the patentee that such a catalyst system may be employed in the further chlorination of chlorobenzene to provide a product high in 1,2,4,5-tetrachlorobenzene. However, although the process disclosed is useful in the further chlorination of chlorobenzenes or in the chlorination of benzene or toluene where a high yield of para-chloro isomer is desired, no indication is seen of the effect of such a catalyst system on the chlorination of dialkyl benzenes such as p-xylene wherein the para position is filled with an alkyl group.
Accordingly, it is an object of the present invention to provide a simple, direct process for the preparation of 2,5-dichloro-p-xylene of high purity whereby the number of processing and handling steps is minimized. It is a further object to provide a process for the catalytic chlorination of p-xylene that maximizes the yield of 2,5-dichloro-p-xylene while minimizing the yield of the unwanted 2,3-dichloro-p-xylene and over- and underchlorinated products.